Method of developing an electrostatic image



Sept. 26, 1961 K. A. METCALFE ETAL 3,

METHOD OF DEVELOPING AN ELECTROSTATIC IMAGE Filed Sept. 24, 1958 UnitedStates Patento METHOD OF DEVEUIJBI IIIISEAN ELEC'I ROSTATIC KennethArchibald Metcalfe, Graymore, South Australia, and Robert John Wright,Hectorville, South Australia, Australia, assignors to The Commonwealthof Australia, care of the Secretary of the Department of Supply,

Melbourne, Victoria, Australia Filed Sept. 24, 1958, Ser. No. 763,016Claims priority, application Australia Sept. 25, 1957 5 Claims. (Cl.117-237) This invention relates to emulsion developers for electrostaticimages.

Development of electrostatic images is at the present time effectedprincipally by three methods, the first of these consisting of the useof dry powders which are dusted onto the image and are subsequentlyfixed by heating or the like, the second method consisting of the use ofa cloud of developer, the third method consisting of the use of liquidswhich carry the developer material.

It has been shownthatthere are substantial advantages in using a liquid,as then the pigments or other developer materials are free to move inliquids of selected characteristic and also fixing of the image can takeplace without added steps such as the heating which was necessaryheretofore.

A liquid developer should preferably be of such a nature that, while itallows free movement of the developer materials to the site where theyare to be deposited, it should not be dangerous from the viewpoint offire or explosion. Most of the liquid developers, however, havecharacteristics such that under suitable conditions they might beinflammable.

The object of the present invention is to improve developers so thatthis risk can be controlled or entirely removed, a further object beingto provide certain im provements to known processes of liquiddevelopment such as more rapid development and better means ofcontrolling the density or tone.

In the dry powder development method already known, a cascade principlehas been used according to, which mechanical objects such as beads arecoatedwith the developer powder and these then apply the powder to theimage as they roll over it, such a method having been found veryadvantageous in methods ofv dry powder development as the developer ismechanically brought into intimate contact with the image irrespectiveof some irregularities in the surface.

The present invention also envisages the use of a carrier phase for thedeveloper, but instead ofusing a solid medium, the carrier is accordingto our invention a liquid or a gas of such surface tension that thedeveloper can be carried on the surface of its globules and can therebybe deposited onto an electrostatic image as the carrier moves thedeveloper in contact with it.

An emulsion developer for electrostatic images according to ourinvention can therefore comprise a liquid carrier phase and a liquiddeveloper phase, the liquid carrier phase being immiscible with theliquid developer phase,

the liquid carrier phase having a surface tension such that the liquiddeveloper phase is carried on the surface of globules of the said liquidcarrier phase when shaken up to produce an emulsion.

According to a more specific form of the invention an emulsion developerfor electrostatic images comprises a carrier phase and a developerphase, said carrier phase consisting of a non-inflammable liquid such aswater, said developer phase comprising a non-polar liquid having avolume resistivity of not less than ohm-centimeter and a dielectricconstant of not greater than three Patented Sept. 26, 1961 saiddeveloper substance having a pigment substance such and immiscible withthe said carrier phase liquid such as v cyclohexane or mineralturpentine or kerosene or xylene,

as phthalocyanine blue suspended in it on the particles of which is acoating forming a control and fixing agent such as a resin or an oilwhich is only sparingly soluble in the developer liquid or remainsdeposited on the surface of the pigment substance when the carrierliquid is evaporated whereby, when the liquids are shaken up to producean emulsion, the globules of the said carrier phase liquid have a filmof the developer phase held thereto by ionic attraction but removabletherefrom by the field of an electrostatic image.

The importance of this will at once be realized when it is appreciatedthat an aqueous or non-inflammable developer carrier can be used, itbeing for instance neces saryonly to combine a developer with waterwhich, when shaken up, will cause the globules of the water tobe coatedwith the developer.

It will be realized that if for instance a substance such as kerosene ismixed with water, a pigment and bonding medium having previously beenincorporated in the kero- -sene according to the method of preparingliquid developer substances, the two phases namely kerosene and waterwill not be miscible, and the effect will be to have a separation whenthe two phases are first mixed together, but when the mixture is shakenthe water will be broken up into globules which will be isolated by afilm of the kerosene base developer, and the size of these globules canthen be controlled by the amount of agitation and in part by therelative ratios of the two phases.

' veloper phase can be carried across the surface by the liquid carrierphase.

The developer tray depicted in FIG. 1 comprises a tray 1 onto which thepaper 2 to be developed is placed, the tray 1 being housed in a casing 3in which the tray 1 is arranged with a slight downward slope to causethe developer to flow across the tray 1 in one direction.

The casing 3 contains a developer tank 4 which carries in it thedeveloper liquid 5 to the level indicated by 6.

To emulsify the two phases of the liquid developer a pump 7 is providedthe rotor 8 of which can be driven in any suitable manner, the pump 7being connected to the developer tank 4 by an intake duct 9 which allowsthe developer to flow into the pump, the developer being displaced bythe pump flowing up the duct 10 to a two-part duct 11-12, the two-partduct being divided by a directmg vane 13 which can be placed either intothe position shown in FIG. 1 or which can be swung over towards theleft.

The part 11 of the two-part duct is disposed to feed liq uid developerdisplaced by the pump 7 back into the developer tank 4 and when themechanism is set in the manner indicated, operation of the pump 7 willcirculate the two phase developer continually through the tank and inpath of the dotted arrows in the duct 12 into the nozzle 8 14 and fromthere will issue across the paper 2 and the tray 1 in the direction ofthe dotted arrows, the developer 3 leaving the tray 1 through the gap 16between its end and the wall of the developer tank 4.

In this way by appropriately positioning the directing vane 13, thedeveloper can be maintained in an emulsified state by circulating thedeveloper through the pump and the developer tank 4 in the direction ofthe full line arrows shown in the drawings, and at this stage no flowwill take place across the tray 1 from the nozzle 14.

A paper to be developed can then be simply placed into position asshown, preferably by engaging its leading edge into a notch 17 of thetray 1 and the directing vane 13 is then simply swung over to the otherposition and the emulsified developer will be pumped through the nozzle14, out of the openings 15 to flow rapidly across the surface of thepaper being developed and the tray and back into the developer tank 4,the directing vane 13 being simply swung back to its other positionimmediately development has been completed which it will be found cantake place in a matter of a few seconds, the paper being then simplyremoved and if an appropriate liquid developer has been used it will befound that the image is dry, or practically dry, the liquid developerphase which coats the aqueous carrier phase globules preventing wettingof the image by water.

It will of course be realized that the actual construction of amechanism for effecting this could be widely varied and in fact it isnot necessary to use a mechanism of the type described as the developercould simply be emulsified by shaking it up in a beaker and immediatelypouring it over the paper to be developed.

Referring to FIG. 2 the paper is again represented by the numeral 2 butthe liquid phase is designated while the developer phase isdesignatedll, the globules of the liquid 20 moving the developer phase21 across the surface 2, it being desirable to have an excess of thedeveloper phase present so that as the film which is held to the waterglobules by the ionic force, which is less than the electrostatic forceon the paper, can be replaced on the surface of the globules duringdevelopment of an image, for it is found that if insufficient developeris present to maintain a coating on the globules 20, that the globulesin contacting the electrostatic image onthe paper will tend to destroythe image as they are then not separated by the insulated coating of thedeveloper from the non-insulating water carrier.

The proportion of the developer phase to the carrier phase can be testedby checking to insure that during development sutlicient globules arenot stripped" of their developer coating to allow these to combine andform larger globules.

The following is an example of howthe invention may be applied.

Developer paste Fifteen grams of a phenol modified penta-erythritolester of rosin such as that known under the trade name Pentarol 20"(acid value 7-15, specific gravity at 20 C. 1.09) is digested intwenty-five grams of xylene and when solution is completed fifteen gramsof a linseed oil modified alkyd resin of medium length such as thatknown under the trade name Rhodene L6/ 100 (oil length 52%, acid value6-10, specific gravity at 20 C., 0.955 to 0.965) is added.

One hundred and fifty grams of phthalocyanine blue is then mixed withthe above mixture and blended either in a ball. mill or triple roll milluntil. mixing is complete and pigment particle size is reduced to therequired fineness, such as 0.5- micron, average. For the most purposeeight hours milling in a ball mill should be sufiicient.

After this it will be found that the pigment particles carry a coating,either continuous or discontinuous, of the coatingmix, whichwillinfluence the surface charge of;

the particles and form the bonding means after'the Pig?- ment is appliedina liquid development process.

'4 Paste dispersion The paste so produced is used as the basis for adispersion in a liquid of high volume resistivity having a zetapotential greater than the ionic potential of the liquid, dispersionbeing etfected by stirring a small portion of the paste into therequired liquid, which in this case may be kerosene, and straining toremove portions of paste that have not completely dispersed.

For fine grain developers filtering should be eifected before use.

Paste concentration The concentration of the dispersion may be varied atwill but a typical concentration of a preferred dispersion is to utilize0.5 gram of developer paste in mils of liquid.

Carrier liquid A liquid developer produced in this way, in whichparticles are coated with a charge controlling and fixing agent which isonly sparingly soluble in the high resistivity liquid or remains as acoating for the particles when the liquid of the developer phase isevaporated, is then mixed in the proportion of 100 cc. of the liquiddeveloper to 100 cc. of water, it being then found that the kerosenebase developer will be immiscible with the water but on agitation thewater will be broken up into globules which will be coated with theliquid developer. The developer is then ready for use in the mannerdescribed earlier herein.

Globule size As a guide to the size of the globules these canconveniently range from approximately of an inch in diameter to /8 of aninch in diameter although these dimensions' are not particularlycritical, but it is found that as the globules are of smaller size, thetotal mixture has a lower volume resistivity with quicker destruction ofthe electrostatic image during development, the particle size thusaffording a degree of control of the time of development, that is thetime during which development can take place before the electrostaticimage is destroyed by the absorption of developer particles and thepresence of the electrically conducting water globules.

Soap emulsifiers and similar material may be used to increase theemulsification provided they are non-ionic, or if ionic, are such thatthey do not reduce the volume resistivity of the mixture below 10ohm-centimeter or the dielectric constant above three. Examples of theseare gum-arable, selenium derivatives, glycol and permanganate.

It will be realized that with a water emulsion as described, the dangerof fire can be materially reduced or completely eliminated because ofthe addition of the noninflammable substance to the inflammablesubstance, and in this way a developer is provided which not only hasimproved developing properties but also materially reduces any firehazard which might otherwise be present, although in practice it hasbeen found that fire risk is negligible even with a carrier base ofrelatively inflammable liquid.

It will be obvious of course that other carriers than Water can be usedand it is interesting to note that apart from the reduction of firehazards there is also a substantial reduction of toxicity if thedevelopers should have this characteristic because the total volume ofthe emulsion developer contains a correspondingly smaller volume of thedeveloper substance.

An effect of these developers is as said, to give good tone control, asthe developer phase can have certain electrical characteristics, whereasthe carrier phase can have other characteristics, and by varying thecharacteristics of particularly the carrier phase it ispossible toregulate the rate of bleed-off ofthe. image charge'fortoning purposes.

Similarly the thickness of the developer coating on the carrierglobules, and the resistivity of the developer liquid itself willprovide further control means.

Front the foregoing it will be realized that the present inventionresides in the use of a developer which forms the phase, the other phasebeing a substance which no miscible with the developer so that thedeveloper ves to coat globules of the carrier, which, as said, can thenact mechanically to convey the developer to the site, while at the sametime controlling the rate of bleedoif of the image charge and serving toreduce or counter the inflammability of the usually more volatile liquidwhich forms the suspending medium for the pigment and the fixing resinor oil or other substance.

The fact that the globules of the carrier phase have deformable wallsallows them to accommodate them selves more readily to surfaceinequalities in the medium being developed and thus greatly improves theprocess over the dry developer and rigid bead system.

It will be clear that the developer must be prepared to insure that thedeveloper material such as pigment, remains in the outer phase where itwill be the contacting medium when the developer is in use, but this canreadily be achieved by first suspending the pigment and resin or otherbonding agent in a liquid of suitable high electrical resistivity andthen adding the liquid or gaseous carrier phase which, on agitation,will form the carrier globules with the developer coating surroundingsame.

To develop an electrostatic image according to this process, theemulsion developer so formed is simply contacted with the image, and thecoated pigment particles which are free to move in the liquid, depositon those areas of the image where a suitable potential exists, until thecharge of the image has been satisfied by the particles deposited, theparticles being held in position by the electrical force until thecarrier liquid is evaporated, whereupon the pigment particles are heldby direct adhesion of the resin or other bonding substance on thesurface of the particles.

In the case of a developer as specifically described herein, thepresence of the resin influence the particle charge, and it is foundthat rapid deposition of the particles is possible when they are socoated.

The coating mix, according to the example is only partly soluble in thecarrier liquid, and therefore, while the partly dissolved surface of thecoating is in a condition where it will form an effective bonding mediumafter evaporation of the solvent, it will act as a surface chargemodifier during development to control the migration of the particals inan electrical field.

The developer of the example can be modified by replacing the RhodeneL6/100 with an equal percentage of Pentarol 20, whereupon it will befound that the developer will have good fixing qualities but will nothave the same degree of modification of the surface charge of thepigment particles as the Pentarol 20 is relatively soluble in thecarrier liquid and is a tacky substance, so that it tends to leave thepigment particles when in the developer and deposit as a fixingsubstance.

If suflicient is present on the pigment substances to cause it to remainthere during development, the develop er can be said to have both acharge modifying agent and a fixing agent.

The developer can also have the Pentarol 20 replaced by an equalpercentage of Rhodene L6/100 a substance less soluble in the carrierliquid specified and the developer then acts as one with a high degreeof control of the surface charge of the pigment particles due to thecoating substance remaining on the pigment particles with possibly evenso little solubility on the surface that no appreciable binding of theparticles will take place after evaporation of the carrier liquid.

What we claim is:

1. A method of developing an electrostatic image which comprisesapplying to a body having a latent electrostatic image thereon anemulsion formed by: mixing a pigment in a liquid to form a developer andplacing the developer in a carrier liquid with which the developer isimmiscible to form an emulsion of the developer and carrier liquid inwhich the developer coats globules of said carrier liquid.

2. A method as defined in claim 1 wherein said carrier liquid isnon-inflammable.

3. A method as defined in claim 1 wherein the amount of developer usedis controlled in order to control in turn communication between theglobules in the emulsion whereby the image is controllably bled off.

4. A method as defined in claim 1 wherein the carrier liquid is waterand the liquid of the developer is kerosome.

5. A method as defined in claim 1 further comprising continuously mixingthe carrier liquid and developer and applying the resulting emulsion tosaid body.

References Cited in the file of this patent UNITED STATES PATENTS1,545,870 Wyllie July 14, 1925 2,171,851 Kipper Sept. 5, 1939 2,537,055Hiron et al. Jan. 5, 1951 2,877,133 Mayer Mar. 10, 1959

1. A METHOD OF DEVELOPING AN ELECTROSTATIC IMAGE WHICH COMPRISESAPPLYING TO A BODY HAVING A LATEN ELECTROSTATIC IMAGE THEREON ANEMULSION FORMED BY: MIXING A PIGMENT IN A LIQUID TO FORM A DEVELOPER ANDPLACING THE DEVELOPER IN A CARRIER LIQUID WITH WHICH THE DEVELOPER ISIMMISCIBLE TO FORM AN EMULSION OF THE DEVELOPER AND CARRIER LIQUID INWHICH THE DEVELOPER COATS GLOBULES OF SAID CARRIER LIQUID.